1. Field of the Invention
The present invention relates broadly to valves for controlling the flow of pressurized fluids and more specifically to flow limit valves that close when the flow rate exceeds a limiting value.
2. Description of the Prior Art
Flow limit valves are commonly used as safety devices in pressurized fluid distribution systems to isolate fluid sources from any ruptures or breaches to minimize the loss of pressurized fluids. Properly designed systems using pressurized gases or liquids that are toxic, corrosive, or explosive employ flow limit valves to minimize exposure to these hazardous chemicals in accident situations. Water distribution systems often utilize flow limit valves to prevent excessive water loss due to pipe breakage.
Functionally, a flow limit valve permits fluid flow up to a predetermined limiting flow rate. The flow rate through a device is proportional to the difference between the upstream supply pressure and the downstream outlet pressure. During normal operation, the pressure differential across the valve establishes a flow rate through the valve that is less than or equal to the limiting flow rate. A rupture in the downstream distribution systems causes a reduction in the downstream pressure and, hence, an increase in the pressure differential across the valve. This increased pressure differential corresponds to a flow rate through the valve which may exceed the limiting flow rate. To limit the flow rate through the flow limit valve to the predetermined limit, a piston, or similar device, is provided which blocks the flow path when the pressure differential exceeds that which corresponds to the limiting flow rate. All flow through the valve is blocked until the pressure differential is lowered by repairing the rupture and the piston is reset to its original position.
A typical flow limit valve in the prior art includes a primary flow path through an orifice from an inlet port to an outlet port. A movable piston is provided to close the primary flow path when the pressure differential across the orifice exceeds a certain value. Fluid from the inlet and outlet sides of the orifice is ported to opposite sides of the piston. The outlet pressure, assisted by a spring, tends to move the piston to an open position, which permits fluid to flow through the valve, and the inlet pressure tends to move the piston to a closed position, which prevents fluid flow. The spring and the piston are designed such that any pressure differential greater than the pressure differential that corresponds to the limiting flow rate allows the inlet pressure to overcome the outlet pressure and the spring force to move the piston to the closed position. To reset this flow limit valve, a bypass valve is opened and fluid flows through a secondary flow path to equalize the pressure on each side of the piston thereby allowing the spring to move the piston to the open position. Certain known valves use a third flow path with an integrated valve to bypass the shut-off piston for providing adjustment of the flow limit. Flow limit valves of these types are expensive to manufacture and difficult to purge due to the multiple flow paths and bypass valves involved. Also, the flow rate through a flow limit valve should be proportional to the pressure differential up to the limiting value of the flow rate and should sharply fall to zero when the limiting value is exceeded. However, spring-biased flow limit valves allow a flow rate that is proportional to the pressure differential up to the point where the piston begins to compress the spring and move from the open position to the closed position, but do not provide a sharp closure because of the additional pressure differential necessary to further compress the spring and complete the movement of the piston from the open position to the closed position.
Further known valves for controlling the flow of toxic or corrosive fluids and gases commonly rely upon flexible members or sliding seals to control operation or reset of the valve from the environment. However, such schemes are generally unsatisfactory because of the danger that the environment may become contaminated by the noxious fluid flowing through the valve if the flexible member or sliding seal fails under fluid pressure. In addition, a flexible member such as a bellows-type or diaphragm-type device or a sliding seal commonly introduces anomalous "pockets" or chambers along the passages for fluids which are difficult to purge of residual fluid when the fluid system has to be cleaned, or a new fluid which must not be contaminated by residual fluid is to be introduced.
What is needed, therefore, is a flow limit valve that is low cost and that provides for a sharp valve closure at a limiting flow rate. What is additionally needed is a flow limit valve that is convenient to reset to an open position and that may be adjusted to different limiting flow rates. Also, such a valve should have minimal "pockets" for easy purging and should obviate the dangers of leaking noxious fluids into the environment through sliding seals.